The study of electric charges Electrostatics

Introduction Did you ever run a comb through your hair? What do you notice. What causes the paper holes to jump onto the comb? There are electrical forces that are in place due to the presence of charge on the comb.

The Atom An atom consists of various charged and uncharged particles. The central region is called the nucleus. Protons (+) and neutrons make up the nucleus. Electrons (-) move around the nucleus in an orbital path. Nucleus Neutrons Protons (+) Electrons (-)

The Significance of Charge An atom with balanced charges is considered neutral. The overall charge can be changed by adding or removing electrons. This makes the atom an ion. Overall Charge: (Neutral Atom) (Positive Ion) (Negative Ion) Add e - Take e -

Sample Problem (Atomic Charge) An helium atom has a net electric charge of -8.0x C. Is it neutral or an ion? Are there extra electrons or a shortage of them? How many extra electrons are there? –Charge Per e - : -1.60* C

Sample Problem (Atomic Charge) An atom has a net electric charge of 4.8x C. Are there extra electrons or a shortage of them? How many electrons short is this atom? Draw this atom given it is Boron.

Electrostatic Demo’s Tape Electroscope

Electric Forces Charges exert a force on other charges Like Charges Repel Opposites Attract

Actual Charge of Protons/Electrons Recall, charge is measured in Coulombs (C). Even though protons and electrons are very small, they still have charge. Let us use q as a variable for charge. Electron Proton

How do atoms get charged? Work can remove electrons from the atom. –Results in a positively charged atom The free Electron can be transferred to another atom. –Results in negatively charged atoms

Separating Charge Charges are balanced in neutral objects. Work must be done to separate charge (free electrons). Once charge is separated, it can be used in experiments.

Separation of Charge Bring a charge rod near a neutral conductor Like charges are repelled Un-like charges are attracted

Charge by conduction A charge rod touches a neutral conductor Like charges are repelled and uniformly distribute

Charge by Induction A charge object is placed near neutral conductors AB ABABAB Separation of charge takes place Contact between the conducting sphere is broken The charge object is removed The charges on the spheres redistribute to maximum separation Result: Two spheres charged by induction

Charging by Polarization Certain substances, such as the one below, have polar molecules. These molecules have opposite charges at each end. Charging by polarization takes place when a charged object is brought near, realigning the molecules in the substance. Magnification

Conductors and Insulators Electrical Conductors are similar to Heat conductors. Electrical Conductors allow charge to move easily. Electrical Insulators do not allow charge to move easily

Conductors and Insulators Electrical Conductors all electrons to move easily. –Metals –Graphite Electrical Insulators do not allow electrons to move easily –Glass –Plastic –Rubber

Coulomb’s Law The electrostatic force one charged object exerts on an other The force is related to the amount of charge –i.e more charge – more force The force is proportional to 1/d 2 –i.e. the further apart the charges, the smaller the force

Coulomb’s Law SymbolUnit FForceN q1ChargeC q2ChargeC dDistancem KconstantN m 2 / C 2

Electro-static Applications Electrostatic filter

F Net Coulomb’s Law in 2-D To find F net with 3 or more charges Calculate each Force vector. It helps to have a grid system on which to work. Use vector addition to find the resultant F net q2q2 q3q3 q4q4 F 13 F 12 F 14 q1q1

Coulomb’s Law in 2-D (cont.) q2q2 q3q3 q1q1 Charge (C) q1q1 3.0 X q2q X q3q3 7.2 X F 12 F 13

Coulomb’s Law in 2-D Sample Determine the direction of each of the forces prior to vector addition. q2q2 q3q3 q1q1 5.4N 0.972N hyp opp adj hyp adj opp Quad II Adjust Quad III Adjust F 12 F 13

The remaining task is to use analytical vector addition. Coulomb’s Law in 2-D Sample MagAngXYQ F N146.3° II F N206.6° III F Net 5.94N154.5° II Quad II Adjust

Electric Field lines Electric Field lines indicate the direction of the force due to the given field. The lines point radially outward from a positive charge. The lines point radially inward from a positive charge.

Electric Field lines Electric Field lines can bend if there is more than one charged particle.

Use symmetry to help.